The present invention concerns the processing of a color cathode ray tube and is directed most particularly to the etching of the color selection electrode of a color tube of the shadow mask variety. In greatest particularity, the invention is addressed to re-etching the shadow mask to enlarge its apertures to a desired predetermined size.
The need for re-etching the shadow mask in color tubes of the type under consideration presents itself when it is desired to have the phosphor deposits on the screen of the tube of smaller dimension than the apertures of the color selection electrode. This requirement is characteristic of both the so-called black surround shadow mask tube and the post-deflection-acceleration or post-deflection-focus color tube.
A preferred form of black surround tube is the subject of U.S. Pat. No. 3,146,368 issued on Aug. 25, 1964 1964 in the name of Joseph P. Fiore et al, and assigned to the assignee of the present invention. In its commercial form its screen is comprised of a multitude of phosphor dot triads each of which has a dot of green, a dot of blue and a dot of red phosphor. Instead of dimensioning the phosphor dots so that they are tangential with respect to one another, the dots are of reduced size so that there is a distinct separation between dots and a pigment or light absorbing material is deposited in those spaces, in effect surrounding each of the dots with a black material. By arranging the holes of the shadow mask to have a larger diameter than the phosphor dots, the electron beams are correspondingly larger than the phosphor dots and the full illumination of the dots permits maximum utilization of the phosphors while the black surround material contributes maximum contrast.
In the post-deflection-focus type of tube the electron beams are subject to a focus field on the screen side of the center of deflection which increases the extent to which electrons of the scanning beams are permitted to impinge on the screen. Because of the post-deflection-focus effect the beams are reduced in diameter and therefore it is desirable to have the phosphor dots smaller than the holes of the shadow mask.
In constructing the shadow mask tube with phosphor dots smaller than the apertures of the mask, it is convenient to form the shadow mask initially with a field of apertures that are sized appropriately for use in photoprinting of the screen. After the mask has been employed in screening, the holes are enlarged to the size, in relation to the size of the phosphor dots, that is desired in the completed tube. Enlargement of the holes may be accomplished by etching if the mask is made of metal such as steel which is normally the case. Since the holes are initially formed in the mask by etching, the enlarging step has come to be known as re-etching of the mask. In accordance with prior practice the re-etching process has been conducted by spraying an etchant over the mask with the intention of having the etchant pass through the holes to effect further etching and the desired enlargement of holes. Previously, the mask has been supported with its apertured portion vertically upward and its frame extending vertically downward but this introduces difficulties in the re-etch process when the spray is directed from above the mask. Such difficulties are avoided by the teachings of the subject invention.